Usually substrates are patterned by the use of lithography. Using this technique the smallest patterns that can be produced are restricted by the resolution of the lithography, namely the wavelength of light emitted by a light source used and the smallest dimensions of the patterning of the photoresist used in the lithography. The smallest dimensions which can be produced by the known 157 nm lithography technology are about 50 nm. However, the effort for achieving this dimension is enormous and thus is only payable when used in patterning substrates in rather big quantities.
Due to this reason, patterning of structures of dimensions less than 100 nm is conventionally achieved with e-beam lithography or focused electron or ion beam technique, since light sources, which can produce light with a wavelength of less than 100 nm, and corresponding photoresists are not available right now. Focused electron beams (FEBs), focused ion beams (FIBs) as well as laser beams and scanning tunneling microscope (STM) probes offer unique advantages over classical resist based processes. Among these are in situ depositions of metals or dielectrics as well as selective etching. Usually, a substrate on which the structures shall be patterned is scanned with an STM and then the substrate is targeted with a focused electron beam or ion beam. All techniques, except laser beams, have resolution capable for nm-sized deposition, i.e. nm-sized dimensions of the produced structures.
The FEB and FIB are usually very expensive. The basic process employed in all the above mentioned techniques is a local Chemical Vapour Deposition (CVD) process at the position of the beam, although the decomposition of the vapour varies strongly with the nature of the input energy. Beside the high costs, a further drawback of the depicted methods is that the substrate has to be conductive or at least some portions of the substrate have to be conductive so that it is possible to start the patterning of the substrate when an STM is used. This imposes a great restriction onto the substrates that can be used.
Another drawback of the above-mentioned approach is that, while scanning the substrate is performed with a low current density, high current densities have to be applied in order to produce an electron or ion beam. Therefore, it is only possible to operate the patterning process in a switching manner, i.e. at one time the scanning is performed with a low current density and at another time the electron beam or ion beam is generated for patterning the substrate. This leads to the problem that after the substrate is scanned, the scanned areas to be patterned have to be found again.
Further, the generation of the electron beam or ion beam is performed in a pulsed operation, since with usually used STMs the necessary high current densities are not achievable in a constant operation. Furthermore, due to the pulsed operation with the high current densities the tip of the microscope wears out quickly.
An alternative technique to define structures in dimensions less than 100 nm is a so-called nanostencil approach. According to this approach, an aperture in a cantilever of an Atomic Force Microscope (AFM) tip is used as a stencil mask in conjunction with a Physical Vapour Deposition (PVD) process. A drawback of this approach is the usage of the PVD that only has a limited operational possibility.
Another method for exposing a photoresist is the usage of a metalized tip of an AFM to enhance the electromagnetic field. According to this method an external light source is used to illuminate a metalized tip with a low intensity UV-light that is not capable to cause photo-conversion. The low intensity UV-light is then enhanced via plasmon resonance to an electromagnetic field of an intensity that is high enough to cause the photo-conversion. Only in the close proximity of the AFM tip the intensity is high enough to convert the photoresist.